qa/vstart_runner: amend ps invocation

qa/vstart_runner: amend ps invocation

"ps -xwwu<id>" is parsed as BSD, because -x is not a UNIX option.
"u" is a BSD option for user-oriented format, so the <id> ends up being
parsed as an old-style "select by pid".  The only reason this command
doesn't dump other user's processes is that the BSD "only yourself"
restriction is in effect.

I'm not sure what's wrong with a simple "ps xww", but if we want to
select by euid, let's do it right.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>